Is there multiplayer or is it co op campaign?
With 300+ hours logged between L4D and L4D2 - I have got my hopes up for this. I had read some middle-of-the-road reviews about it and that some people mentioned that it was 'lacking' something.
As @Greeny. says - I'll probably nab it when its cheaper and/or DLCs have been released. The current price does seem really quite high.